I thought the numbers were one of the things that made it fun. Having a low ICQ number showed you were an early adopter, much like having a short URL or twitter name.
Yeah, fun for the million or so first users, not so much for the next 2 billion.
I specifically remember hating those numbers (because I always forgot mine) and being spitefully relieved to switch to MSN Messenger when that came out.
Yeah, it made it fun for the early adopters, and less fun for everyone else. That's not much of a strategy, considering how few early adopters there are.